1. Field of the Invention
The present invention relates to a method of setting a reverse activity bit in a mobile communication environment, and more particularly, to a method of setting a reverse activity bit for enabling subscribers in a cell or sector to receive services and to increase a reverse link capacity.
2. Discussion of the Related Art
Generally, 1xEV-DO is a system for exclusive data communication developed by Qualcomm Inc., U.S.A. in the late 1990's to provide general data communication services in non-realtime wireless mobile environments. The 1xEV-DO system adopts intrinsic resource assignment methods corresponding to the characteristics of forward and reverse links.
In a forward link, a base station continuously transmits pilot signals with a constant power. Each terminal in a cell measures intensity of each of the pilot signals to determine the channel status of the forward link. Once such a status is reported to the base station, a scheduler of the base station determines which terminal will receive an assignment of a forward link resource.
This determination is made in accordance with the forward link channel status reported from each of the terminals in every slot with time division. The scheduler operates so that more resources are assigned to the terminal with a better channel status. This maximizes total data throughput of forward link for each cell.
In a reverse link, a base station receives reverse pilot signals from terminals scattered in a cell. The base station is unable to check reverse link characteristics of the respective terminals from reception signal intensity received from each of the terminals via an antenna of the base station.
Instead, the base station measures intensity of total reception signals received via its antenna, thereby indirectly controlling a load amount in reverse link by directing reverse link resource assignment of the same level to all terminals based on the measurement value.
Each of the terminals in the cell receives reverse activity bit (hereinafter abbreviated ‘RAB’) by each slot from the base station, and the terminal then determines a data rate of next frame with probability based on a value of the received RAB.
If the RAB received from the base station is to lower the data rate, each of the terminals determines whether to lower or maintain the data rate of the next frame with probability according to the current reverse link data rate. If the base station directs to raise the data rate, each of the terminals determines the data rate of the next frame by the same method.
A reverse traffic channel has six kinds of transmission rates shown in Table 1.
TABLE 1Transmission rateMaximum payload(Kbps)(bits)009.623419.249038.4100276.82026153.64076
The terminal uses one of the transmission rates in Table 1 for transmission. In this case, the terminal enables to raises or lowers the transmission rate of reverse traffic channel using the information (RAB) received from the base station.
Thus, each of the terminals in the cell adjusts data rate of reverse link according to RAB set by the base station. RAB setup directly becomes a means for adjusting the load amount of reverse link.
Qualcomm Inc. has proposed two methods for setting RAB, and the first method is recommended.
The first method uses thermal noise power vs. reception signal power (hereinafter abbreviated ROTm). The ROTm is a measurement value (dB basis), which is found by subtracting thermal noise power (Rx Power_Thermal) of a base station system from total reception power (Rx Power_Total) received from an antenna of a receiving-end of base station, for judging a load status of reverse link. ROTm is periodically measured by a RF device in the base station.
If ROTm measured in a receiving-end of an access network per each slot (1.67 ms) for each sector and antenna is higher than a reference value (ROTm_th, 5 dB recommended), RAB is set to ‘1’ to direct to lower transmission data rate of the terminal. If ROTm is lower than the reference value, RAB is set to ‘0’ to direct to raise data rate of the terminal. A basic setup value of RAB is basically ‘0’.
Meanwhile, the second method uses a calculated ROT value (hereinafter abbreviated ‘ROTc’). A reverse load can be found by the second method using an equation. Firstly, a ratio of reception pilot power vs. total reception power is found for each reception data rate using variables such as DataOffsetNorm, DataOffset9k6(9,600).
ROTc that means a reverse link load of a corresponding sector can be found by multiplying a total of f(DataRate) values of the entire access terminals in the sector by Ecp/Io [average energy per chip of pilot channel in antenna vs. PSD(power spectral density) of total reception signals]. Yet, such an equation fails to consider power by ACK channel. The related art method of setting RAB (reverse activity bit) in a communication system has the following problems or disadvantages.
In case of a terminal for 1xEV-DO, since data rate of next frame is determined by probability according to RAB received from a base station, the base station is unable to accurately control or determine a reception load amount in the next frame or a reception signal power of an antenna end despite setting and transmitting the RAB to the terminal.
Second, in case two terminals located close to a base station, with a moderate frame offset difference therebetween, detect RAB to increase data rate, and determine to simultaneously increase their data rates and transmission power, total reception signal power is increased. Also signal vs. noise ratio for each call is degraded, and the terminals competitively raise their transmission power by fast closed loop power control. All of the terminals in the cell will then transmit with maximum power as high as they can.
If such a situation takes place under a ROT value lower than a reference value (ROTm_th), the transmission power is unnecessarily increased. This causes extra power consumption and adversely influences the reverse link of a neighboring cell.
In order to overcome the second problem, the reference value (ROTm_th) is compared to the measured ROT value. If the reference value is lower, the number of times of RAB commands directing the terminal to lower the transmission data rate is increased before a reception capacity limit is reached. This results in reducing the reception capacity of reverse link.
Moreover, since the accuracy of ROT measured by a base station of a current mobile communication system such as 1xEV-DO system is not reliable yet, the related art method of setting RAB by simply comparing the measured ROT value to the reference value ROTm_th has various limitations in many aspects. Hence, a new method enabling to overcome such limitations is needed.